Combined Transcranial Magnetic Stimulation and Electroencephalography of the Dorsolateral Prefrontal Cortex

TMS-EEG enables the direct and noninvasive stimulation of most cortical areas and the acquisition of the resulting neuronal activity with very good spatio-temporal resolution³⁰, especially when neuronavigation is utilized. The benefit of this methodological advance is based on the fact that TMS-evoked EEG signals originate from the electrical neural activity and it is an index of cortico-cortical excitability. This has tremendous potential in neuropsychiatric patient populations where TMS-EEG can be used as a biomarker of current and future therapeutic interventions.

The most critical step of the protocol is the preparation of the electrodes and the determination of the stimulation intensity. This is because the TMS-EEG signal is susceptible to the TMS artifact, irrespective of the kind of amplifiers used³¹. The electrodes should be very carefully prepared, so they do not bridge with each other and their impedance is kept below 5 kΩ, and the signal-to-noise ratio is high. In addition, a sponge made from artificial polyutherane fiber of 5-10 mm adjusted under the coil can further reduce the mechanical pressure and the artifact of the coil click sound through bone conduction.

The MT determines the TMS-intensity; therefore, it should be measured precisely as higher intensities will lead to bigger artifacts and less focal stimulation, while smaller intensities may result in very weak signals. Thus, the motor hot spot should be found with the assistance of neuronavigation and the MT is estimated with EMG recordings (noise below 50 µV and muscles completely relaxed). However, one should not forget that the focality and the accuracy of each stimulation derives from the shape and duration of the TMS pulses³².

The lack of measures for a DLPFC threshold also suggests that the intensity should be adjusted according to the amplitude of the estimated induced electrical field²³ and not based on the stimulator's intensity output as the conventional method. This requires that the MT intensity needs to be estimated in V/m for a specific cortical depth and then the same depth and V/m to be used to recalculate the stimulator's output intensity for DLPFC stimulation. This is a particularly important issue for future investigation of paired pulse protocols like the ones presented here, where the TS is always at suprathreshold intensities. However, there is a need to define the DLPFC intensity from the recorded TEP³³ or oscillations³⁴ during DLPFC stimulation as it has been suggested in recent studies for M1 by means of cortical and non-corticospinal measures.

Importantly, the DLPFC stimulation site should be chosen based on the MNI or Talairach coordinates and inserted on the MRIs of the neuronavigation. The MNI coordinates for the left DLPFC (-35, 45, 38) are drawn from a study identifying this site as optimal, based on clinical outcomes and resting-state functional connectivity³⁵. The positioning of the coil with respect to orientation and tilt is another important variable. There are two ways to approach coil orientation and tilt: a) 45 degrees to the midline with the handle pointing to the lateral parts of the hemisphere⁹ and b) perpendicular to the middle frontal sulcus with lateral to medial current direction¹⁴. The first is usually applied when no navigation exists, while the second one requires real MRI and navigation and it induces the maximum of the field. Before starting the recordings, fine tuning of the coil so it evokes minimum muscle artifacts⁵ without affecting the stimulation physiological responses needs to be performed (small changes of 1-2 mm of the center of the coil, as well as tilt and orientation subtle changes).

Comparison of the different orientations needs to be done since there are no known studies that have examined the effect of different coil positioning over DLPFC. Even more importantly, there is a need of a method to define the DLPFC hot spot based on EEG measures in a similar manner that the M1 hot spot is defined by the EMG. Finally, a very important aspect here is the positioning of the electrodes and their digitization of their location. In test-retest designs, as soon as the cap is placed for the follow up experiments, the electrodes should be digitized. Then both digitizations (of the first and the consequents experiment) should be visualized over the 3D MRI model or the MRI template (which can be a good reliable solution when individual MRIs cannot be obtained). Then the cap should be moved if necessary so the positioning over the skull of the electrodes in the follow up experiment matches the positioning of the first measurement. This will ensure that the data will be derived from the exact same locations of the electrodes that were stimulated with the exact same magnetic field.

Before starting the stimulation, the chosen cortical site should be checked for cranial nerves passing under the coil. Therefore, a few TMS-EEG epochs should be recorded, and the artifacts evaluated. Thus, signal needs to be checked for amplitudes larger than 70 µV and non-synchronized high frequency-low amplitude oscillations (muscle and cranial nerves artifacts). Eliminating such artifacts can be done by fine and subtle repositioning of the coil or its orientation only, as it has been proposed in previous studies³⁶. Finally, during the TMS-EEG sessions, the TMS coil should be monitored by real time neuronavigation and kept immobilized. The best way is to mount it on a tripod or on a mechanical arm. This solution also prevents from pressing the coil with the hands against the electrodes, adding mechanical pressure artifacts on them. Any changes should be immediately corrected and the respective epochs marked as bad and excluded from the data analysis, due to the fact that EEG responses to TMS are very sensitive to the perturbation of these parameters³⁷. All these detailed suggestions can ensure test-retest reliability of TMS-EEG in single¹⁴ and paired pulse paradigms¹⁵ over the DLPFC. The attention to these important details will ensure that the data have the highest chance of reflecting changes related to the therapeutic interventions.

TMS-EEG like every other experimental method has its own specific limitations. The major issue is the various kinds of artifacts and the fact that TMS-compatible EEG amplifiers cannot eliminate the remaining artifacts. Artifacts from cranial muscles, particularly when frontal and lateral sites over the skull are stimulated, can obscure and modulate the EEG signal. These artifacts can be larger than the TMS-EEG signal and usually last longer, thus they may obscure the TEPs. Similarly, but only in areas such as the DLPFC, TMS can evoke large eye blink artifacts. Additionally, many other artifacts such as electrode movement, skin sensation and auditory activations due to TMS coil click can make the EEG analysis even more difficult (for details, see previous publications^31,38). Much work in the field has been directed towards rejecting a variety of artifacts, resulting in more reliable spatio-temporal localization of the sources of the brain responses^{38,39,40,41,42}. However, one should not forget that the careful preparation of the participants, the choice of the equipment and accurate performance of the measurement determine the quality of the raw TMS-EEG data.

TMS-EEG is a powerful tool to assess intracortical inhibition and excitation mechanisms related to the stimulation of the DLPFC. By just changing a few parameters, it allows for the study of circuits mediated by GABA_AR (SICI), GABA_BR (LICI), and NMDAR (ICF). Modulation of different TEP components through pharmacological or electromagnetic therapeutic interventions can serve as a marker to identify inhibitory and excitatory neurotransmission, cortical plasticity and many more brain state changes and conditions⁴³. In addition to TEP's, TMS-evoked oscillatory activity through time frequency and spectral analysis can assess the natural or the intrinsic frequency of the above circuitry¹⁰. Electrical brain indices such as current source density⁴ applicable for any cortical area may help to unravel the mechanisms of plasticity in damaged brain circuits in DLPFC⁴⁴.

Further pharmacological validation studies of these paradigms in the DLPFC are necessary. However, there is tremendous potential for TMS-EEG to be used to study the mechanisms of various therapeutic interventions, such as neuromodulation therapies (e.g., rTMS, ECT, MST) or pharmacological ones in healthy volunteers or in various psychiatric disorders^{9,15,16,17,45,46}, but also alternative interventions or combinations of them⁴³. Most importantly, TMS-EEG can reliably assess the brain dynamics prior to and after an intervention and therefore potentially serve as biomarker.